PreVote capability in raft.h and tracing for it

[cjen1-msft]

This PR makes raft.h capable to respond to pre-votes, but does not enable the PreVoteCandidate by default.

#7361

To be specific about the changes, this PR should largely be a noop for the existing tests as it does not enable the pre-vote by default and we have no way to do so manually.
(Whether/how to enable pre-vote is very much so up for debate)

If pre-vote is enabled, there are the following changes.

• periodic
  • If pre-vote-enabled then become a PreVoteCandidate and send RequestVote{.is_pre_vote = true}
• recv_request_vote
  • If m.is_pre_vote then don't update state
  • Note: will always update the term if necessary (see comment)
• recv_request_vote_response
  • Guard clauses for receiving a pre-vote response when already a candidate

There is a new trace_scenario.sh to make trace validation for a single test simpler.

Also updated ccfraft.tla to make preVoteStatus per-node rather than a global per-network config.

My aim for this PR is to nail down the raft.h changes wrt trace validation, with the expectation that no end-to-end tests should change (as nothing is enabled yet).
And this will be the PR that gets backported to 6.X (hopefully).

[Copilot]

Pull Request Overview

This pull request implements the PreVote extension for the Raft consensus protocol to prevent disruption from network-partitioned nodes that can send RequestVote messages but cannot hear AppendEntries from the current leader.

Key changes:

• Introduces a PreVoteCandidate leadership state where nodes first request speculative pre-votes before starting actual elections
• Changes preVoteStatus from a global set to a per-server function mapping each server to its PreVote capability set
• Adds is_pre_vote flag to RequestVote and RequestVoteResponse messages to differentiate pre-vote from actual vote requests

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tla/consensus/ccfraft.tla	Changes preVoteStatus to per-server mapping and updates all references to use indexed access
tla/consensus/Traceccfraft.tla	Updates trace validation to handle PreVoteCandidate state and per-server preVoteStatus
tla/consensus/SIMccfraft.tla	Updates simulation initialization to use per-server preVoteStatus with consistency constraint
tla/consensus/MCccfraft.tla	Updates model checking initialization to use per-server preVoteStatus with consistency constraint
tla/consensus/MCAliases.tla	Adds preVoteStatus to debug output and enables trace debugging
tests/raft_scenarios/pre_vote_split	New test scenario validating PreVote behavior with mixed PreVote capabilities
tests/raft_scenarios/pre_vote	New test scenario validating basic PreVote election and migration behavior
src/kv/kv_types.h	Adds PreVoteCandidate to LeadershipState enum
src/consensus/aft/test/driver.h	Adds PreVote configuration support and fixes detail value extraction
src/consensus/aft/test/driver.cpp	Adds pre_vote_enabled command to test driver
src/consensus/aft/raft_types.h	Adds is_pre_vote field to RequestVote and RequestVoteResponse messages
src/consensus/aft/raft.h	Implements PreVote logic including become_pre_vote_candidate and vote handling
src/consensus/aft/impl/state.h	Adds pre_vote_enabled field to State structure
scripts/trace_scenario.sh	New script to build, run, and validate Raft scenarios against TLA+ spec
doc/architecture/consensus/index.rst	Adds comprehensive documentation of PreVote extension and migration strategy